Wrong result (or questionable result and behavior) with aggregate function in uncorrelated SELECT subquery

MariaDB [test]> INSERT INTO t2 VALUES (3);

Query OK, 1 row affected (0.00 sec)

MariaDB [test]> --echo # This looks wrong. t1 has 2 rows, so the result set should have 2 rows too,

MariaDB [test]> --echo # but it only has 1:

MariaDB [test]> SELECT ( SELECT MAX(f1) FROM t2 ) FROM t1;

+----------------------------+

| ( SELECT MAX(f1) FROM t2 ) |

+----------------------------+

|                          2 |

+----------------------------+

1 row in set (0.00 sec)

MariaDB [test]> 

MariaDB [test]> --echo # This also looks wrong. Now there are two rows in the result set,

MariaDB [test]> --echo # but they are different, which should not happen, as we select MAX:

MariaDB [test]> SELECT ( SELECT MAX(f1) FROM t2 ) FROM v1;

+----------------------------+

| ( SELECT MAX(f1) FROM t2 ) |

+----------------------------+

|                          1 |

|                          2 |

+----------------------------+

2 rows in set (0.00 sec)

MariaDB [test]> 

MariaDB [test]> INSERT INTO t2 VALUES (4);

Query OK, 1 row affected (0.01 sec)

MariaDB [test]> 

MariaDB [test]> --echo # This shouldn't even work, because the subquery returns more than 1 row,

MariaDB [test]> --echo # but it works and returns wrong result, same as before:

MariaDB [test]> SELECT ( SELECT MAX(f1) FROM t2 ) FROM v1;

+----------------------------+

| ( SELECT MAX(f1) FROM t2 ) |

+----------------------------+

|                          1 |

|                          2 |

+----------------------------+

2 rows in set (0.00 sec)

MariaDB [test]> 

MariaDB [test]> --echo # This returns the error as expected:

MariaDB [test]> SELECT ( SELECT MAX(f1) FROM t2 ) FROM t1;

ERROR 1242 (21000): Subquery returns more than 1 row

--disable_abort_on_error

CREATE TABLE t1 (f1 INT);

CREATE VIEW v1 AS SELECT * FROM t1;

INSERT INTO t1 VALUES (1),(2);

CREATE TABLE t2 (f2 int);

INSERT INTO t2 VALUES (3);

--echo # This looks wrong. t1 has 2 rows, so the result set should have 2 rows too,

--echo # but it only has 1:

SELECT ( SELECT MAX(f1) FROM t2 ) FROM t1;

--echo # This also looks wrong. Now there are two rows in the result set,

--echo # but they are different, which should not happen, as we select MAX:

SELECT ( SELECT MAX(f1) FROM t2 ) FROM v1;

INSERT INTO t2 VALUES (4);

--echo # This shouldn't even work, because the subquery returns more than 1 row,

--echo # but it works and returns wrong result, same as before:

SELECT ( SELECT MAX(f1) FROM t2 ) FROM v1;

--echo # This returns the error as expected:

SELECT ( SELECT MAX(f1) FROM t2 ) FROM t1;